Electrophotographic "laser" printers, wherein a modulating, scanning laser is projected onto a photoconductive surface to create an image to be printed, are well known. In the case of printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of optical patterns to be imaged on photographic film or an electrostatically charged photoreceptor (a photosensitive plate, belt, or drum) for purposes of printing. The ROS provides a laser beam which switches on and off according to electronic image data associated with the desired image to be printed, exposing the charged photoreceptor point by point as the beam moves, or scans, across its surface. Commonly, the surface of the photoreceptor is selectively imagewise discharged by the laser beam in locations to be printed white, to form the desired image on the photoreceptor. A common technique for deflecting the modulated laser beam to form a scan line across the photoreceptor surface uses a motor-driven rotating optical polygon with multiple reflecting surfaces; the laser beam from the laser source is reflected by the facets of the polygon, creating a scanning motion of the beam, forming a sharply focused scan line across the photoreceptor surface. A closely spaced regular array of scan lines on a photoreceptor collectively forms a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with toner, and the developed image is transferred to a print sheet, as in the well-known process of electrophotography.
Recently there has been proposed a general design for an electrophotographic printing apparatus which renders full-color images. Under this proposed architecture, successive layers of different primary-colored toners (typically, black, cyan, magenta, and yellow) are "built up" in imagewise fashion on the same general area of a photoreceptor. When all four different-colored layers are thus placed on the photoreceptor, the set of different-colored toners are in one operation transferred to a sheet of paper, rendering a full-color image thereon. In this architecture, generally known as image-on-image (101) architecture, it will be necessary that a latent image be created on a photoreceptor which will already have at least one layer of toner, from a previously-placed primary-color image, thereon. Thus, the laser which discharges pixel-sized areas of the photoreceptor to render a particular image must pass through one or more previously-placed toner layers in order to discharge a particular small area of the photoreceptor.
The discharging effect of a laser should be uniform whether or not the laser is passing through one or more layers of toner, which in turn means that the intensity of a laser should be increased in areas where the laser must pass through one or more layers of toner. Previously-placed toner will interfere with the transmission of light from the laser to the charged surface of the photoreceptor. Exactly where, within an image to be printed, one or more layers of previously-placed toner will be for a particular portion of a scanline is ultimately dependent on the exact nature of the image being created, and will of course change depending on what specific portion of the image is being printed. There therefore exists a need to monitor and adjust the intensity of a laser discharging any particular portion of a photoreceptor in an 101 color printing apparatus.